Silicon thermal oxide films are used as gate insulating films of MOSFETs for memory, capacitor insulating films of DRAMs, and the like. With the recent and continuing increase in the integration degree of semiconductor devices, it is necessary to reduce the area occupied by a MOSFET and the like. For that purpose, it is required to reduce the film thickness of a silicon thermal oxide film so as to maintain a constant capacitance. In addition, due to scaling arising from miniaturization of devices, nowadays it is required to reduce the film thickness of the thermal oxide film to several dozen Å. Further, the same applies to the case of forming a thermal oxynitride film instead of a thermal oxide film.
Such reduction of the thickness of a silicon thermal oxide film or a thermal oxynitride film causes a direct increase of the tunnel current. Hence, there are problems in that circuits of semiconductor devices do not operate normally or power consumption is increased since a leakage current is produced.
Consequently, a silicon nitride film or an oxynitride film, having a dense structure, is considered as a good insulating film as an alternative to a silicon thermal oxide film or a thermal oxynitride film.
Such silicon nitride film or oxynitride film is formed by nitriding or oxynitriding a silicon thermal oxide film or a thermal oxynitride film. Then, by achieving a capacitance with a relatively great dielectric constant of the nitride film or the oxynitride film, it is possible to increase the film thickness (physical film thickness) of a nitride film or an oxynitride film having the constant capacitance as that of a silicon thermal oxide film that maintains a constant capacitance. Thereby, reduction of the leakage current is achieved. Hereinafter, in this description, the thickness of a silicon nitride film or an oxynitride film converted to the thickness of a silicon thermal oxide film that gives an equivalent capacitance is referred to as an electric film thickness.
Incidentally, in the silicon nitride film or the oxynitride film formed as mentioned above, it is not necessarily easy to control the film thickness and the uniformity within the surface of the film with accuracy.
For this reason, a method is considered in which, by using the ALD (Atomic Layer Deposit)-CVD method, an operation of forming a monatomic layer or a monomolecular layer of a silicon nitride or an oxynitride is repeated to deposit a plurality of monatomic layers or monomolecular layers, thereby forming a film of a predetermined thickness.
However, generally, the method using the ALD-CVD method, which is considered currently, requires about several dozen minutes, for example, to form one monatomic layer or monomolecular layer. Additionally, in order to form the predetermined thickness, it is necessary to repeat the operation of forming a monatomic layer or a monomolecular layer several dozen times. Hence, there is a problem in that production efficiency is significantly lowered.
A specific example of the ALD-CVD method will be described.
First, a silicon substrate is subjected to thermal processing in a nitrous oxide gas, thereby forming a silicon oxide film having a film thickness of 12 Å (angstrom) on the silicon substrate. Then, a process is performed at the temperature of approximately 400° C. by using TCS (tetrachlorosilane) as a process gas, and one monomolecular layer of TCS is formed through adsorption. Thereafter, the monomolecular layer of tetrachlorosilane is processed at the temperature of approximately 550° C. by using ammonia gas as a process gas so as to form one monomolecular layer of a silicon nitride (nitride of silicon). Then, in order to obtain the film thickness of, for example, 15-20 Å, the forming operation of the monomolecular layer of the silicon nitride is repeated for, for example, twenty times.
It is reported that the uniformity within the surface of a film is significantly improved according to the above-mentioned ALD-CVD method.